Electric temperature control system



Sept. 19, 1950 F. J. HOOVEN 2,522,826

ELECTRIC TEMPERATURE CONTROL SYSTEM Filed Feb. 20, 1947 Cam/real. \r\ COMPEMSHTOE BLflA/KET I c 27 I A I Y B Y l I TI L 1 I 0c 3 a I Pu I I l 7 I l l 2 I '7 fl! l l l .1 9 fL fx /5 a I flMPuF/e'e c SflFETY Y 0 Pas/awed a? zany Patenteci Sept. i9, 1950 ELECTRIC TEMPERATURE CONTROL SYSTEM Frederick J. Hooven, Dayton, Ohio, assignor, by mesne assignments, to Clarence E. Lapedes,

Dayton, Ohio Application February 20, 1947, Serial No. 7 29,686

15 Claims.

This invention relates to an electric temperature control system and is designed more particularly for use with an electrically heated blanket, or the like.

One object of the invention is to provide such a control system which is highly efficient in operation, simple in construction and relatively in expensive to produce and operate.

A further object of the invention is to provide such a control system in which the heating element is maintained at all times, when the system is in operation, at a temperature closely approaching the temperature for which it is set.

A further object of the invention is to provide such a control system which is responsive to ambient, or room, temperatures.

A further object of the invention is to provide such a control system with an amplifier which positively and accurately controls a relay in the heater circuit.

A further object of the invention is to provide such a control system which is entirely safe in operation and in which no adverse effect can result from the accidental breaking, or otherwise opening, of the heater circuit.

Other objects of the invention may appear as the system is described in detail.

In the accompanying drawings, Fig. l is a circuit diagram of the system; and Fig. 2 is a diagrammatic illustration of the amplifier.

In these drawings I have illustrated one embodiment of my invention and have shown the same with a preferred form of circuit and with a preferred type of amplifier, but it is to be understood that the circuit and the amplifier may take 1.

various forms, and that the control system may be used for various purposes, without departing from the spirit of the invention.

In the form here shown the main circuit comprises conductors 5 and 6 connected respectively with terminals A and B, whereby the system may be connected with an alternating current power line, as by plugging the same into an ordinary electric outlet. This main circuit includes a bridge circuit one leg of which constitutes a resistance heater Rh, which may be of any character suitable for its intended use but which, when adapted for insertion in a blanket, is usually of substantial length and is distributed through a large portion of the blanket. The heater Rh is connected by conductor 1 and resistor RI with conductor 6, and through that conductor is connected with resistors Re and Ra. The resistor Re is connected through a conductor 8 and resistor R2 with a conductor 9, which in turn is connected with the conductor II], which is corinected with that end of the resistor Rh opposite the end with which the conductor 1 is connected. The conductor 8 is also connected with an amplifier X and the conductor 1 and resistor RI are also connected with the amplifier by a conductor I l. The resistor RI being between the heater and the terminal B should be of relatively small value and the opposed resistor R2 should be of a correspondingly large value. The resistors Re and Ra constitute that leg of the bridge which is opposed to and balances the'heater Rh. The resistor R0 is a variable resistor which can be manually adjusted to determine the temperature at which the heater Rh is to be maintained. The resistor Rd is responsive to ambient, or room, temperature, and is, therefore, preferably wound of wire having a high positive temperature coeificient, such as iron, nickel, or copper. The two resistors, Re and Ra, are preferably mounted in a control box l2 spaced from the heater and the other elements of the system so that they will receive no heat from such elements but the resistor Ra will respond to changes in the ambient temperature and thereby vary the resistance of the heater Rh in accordance with the variations in the room temperature. The heater circuit is connected with the terminal B through a relay, one switch member l3 of which is connected with the conductor 9 and the other switch member I4 of which is connected with the conductor 5. The relay winding is shown at I5.

The relay is controlled by the amplifier X which is preferably of such a character that the relay will operate to maintain the temperature of the heater Rh substantially constant while the system is in operation. This is preferably accomplished by so controlling the operation of the relay that the heater circuit will be closed at frequent intervals regardless of the temperature of the heater. For this purpose I employ two or more vacuum tubes, the plate circuit of one of which includes the relay winding, and the grid bias of that tube being controlled by means connected in the plate circuit of a second tube, the output of which is responsive to variations in the resistance of the heater R In the illustrated amplifier I have provided three tubes designated respectively as VI, V2, and V3. The cathode N5 of tube V3 is connected by a conductor ll with a terminal A, and the plate I8 of tube V3 is connected with terminal B through conductors I9 and 6, the winding l5 of the relay being included in the plate circuit. The cathode 20 of tube V2 is connected with terminals A and B through a resistance divider comprising a part R3 connected with the conductor I1 and a part R4 connected with the conductor 6. The plate 2I of tube V2 is connected with the terminal A through the conductor 22, a resistor R6 and the conductor H. A condenser CI is connected between the conductors 22 and IT in parallel with resistor R6, and the condenser CI and the conductor 22 of the plate circuit are connected with the grid 23 of the tube V3. The cathode 24 of tube VI is connected through a conductor 25, and resistor R2 with the terminal B. The plate 26 of tube VI is connected by a conductor 2'! with the grid 28 of tube V2 and is also connected through a resistor R and conductor I! with the terminal A. The grid 29 of tube VI is connected with the heater Rh and resistor RI of the bridge circuit through a resistor 30, the resistor 30 being added to prevent grid current flow during opposite cycles of the line voltage. The cathodes of the tubes may be heated in any suitable manner and in the present instance each tube is provided with a separate heater 3|, which is connected in the main circuit as by conductors 32 and 33 as shown in Fig. l. The tubes VI and V2 being connected in cascade and the cathode of tube V2 being connected with the resistance divider R3, R4 across the line and both tube plate circuits being connected with terminal A of the alternating current line, the tubes VI and V2 will draw current only when terminal A is positive, and the plate circuit of the tube V3 being connected with the terminal B will draw current only when terminal B is positive. The two parts of the resistance divider R3 and R4 are of approximately the same value, and during the brief interval, or one-half cycle, that terminal A is positive the cathode of tube V2 will be positive with respect to the oathode of tube VI by approximately one-half the line voltage, and since resistors RI and Re plus Ra are small in value in comparison with the heater Rh and resistor R2 the cathode of tube VI has substantially the potential of terminal B.,

Therefore, assuming that the line has a voltage of 120 volts, and that the resistance of RI is 5 ohms and the resistance of the heater Rh ,is

connected with the grid 23 of tube V3, and the condenser being of large capacity as compared with the resistor R6 will remain charged during the interval that the alternating current voltage is changing its polarity. Thus, when terminal 13 is positive, tube V3 will draw current from the line and that current will decrease as the charge across condenser Cl increases. As the resistance in the heater Rh increases and its temperature rises, the output current of tube V3 will decrease until it is insufficient to retain the relay closed. After the relay has opened, there will be no current flow through the plate circuit of tube V2 and condenser CI will discharge across resistor R6, and when. the charge has been reduced to a predetermined value plate iii of tube V3 will again draw current in sufiicient quantity to close the relay, which will then remain closed until the temperature of the heater reaches the value for which it is set.

In the present apparatus the values are such that, the tube V3 will draw current suflicient to close the relay when the voltage across condenser CI and resistor R6 is 10 volts or less, and the relay will remain closed until the voltage increases to 20 volts or more. It may be assumed that the voltage across condenser CI and resistor R6 is 20 when the grid of tube VI has a negative bias of 1 volt and that the voltage across the condenser CI and resistor R6 is changed by an amount equal to one thousand times the change in the grid voltage of tube VI. Under such conditions each 1.0 ohm change in the value of re- 75 ohms, the grid of tube VI will be 7 volts posino negative bias and tube VI will be fully conducting. Thus there will be a large voltage drop across resistance R5, and the plate of tube VI and the grid of tube V2 will be negative withv respect to the cathode of tube V2. Tube V2 will then be non-conducting and no current will flow through.

resistance R6 to the plate of tube V2 and the condenser CI will discharge through resistance R6. When the relay is closed and the grid of tube reaches that value.

sistance of heater Rh will cause a change of current of /80 through heater Rh and resistor RI which will in turn cause a change of 7 /2/ 80 or .094 volt in the grid voltage of tube VI. Thus each .1 ohmvariation in the resistance of heater Rh causes a change of about 9.4 volts across condenser CI and resistor R6. If heater Rh has a positive resistance-temperature coefficient of .00133 ohm per degree Fahrenheit, this .1 ohm change in a 75 ohm heater corresponds to about 1. F. change in temperature.

The capacity of the condenser CI as compared to resistor R6 will, of course, be determined according to the required operation, and in the present instancejthe proportions are such that about twenty seconds are required for the voltage across the condenser and resistor to decrease from 20 volts to 10 volts. The resistance of heater Rh being '75ohms atthe desired temperature, the relay will open with 20 volts across the condenser and resistor when the resistance of heater Rh The relay will the remain ,open for the, twenty seconds required for the VI has a positive voltage of say 7 compared with volts, the grid of tube VI will have a negative Under such conditions the voltage of 1 volt.

voltage across the condenser and resistor to drop to 10 volts, since there is no grid bias on tube VI and no current will be drawn by the plate of tube V2. During this twenty seconds the heater will have cooled ofi slightly so that when the relay again closes it will stay closed until the heater has again reached the temperature for which it has been set. The length of time required to do voltage drop through resistor R5 will decrease be- As has been above explained, condenser Cl is this will be dependent upon the amount that the heater Rh has cooled during the twenty second interval. If the heater Rh has not cooled appreciably during the twenty second interval, the relay will remain closed only long enough for the condenser CI to charge from 10 volts to 20 volts through the plate circuit of tube V2. If the re- 'sistance of R6 is 500,000 ohms and the plate resistance of tube V2 is 40,000 ohms then the charging of the condenser will require only 1.6 seconds. Thus the minimum current the controller is able to put into the control circuit is about 8% of the maximum value. In a 150 watt blanket this is about 12 watts on the average, which is scarcely perceptible to the sleeper, whose heat radiation amounts to about 60 Watts.

In practice the value of resistor R2 is about one thousand times that of the heater Rh, that is, approximately 75,000 ohms, and resistor R plus resistor Ra is likewise one thousand times that oflresistor RI, or about 5,000 ohms. Therefore, if the heater Rh changes 1 ohm for each 1 of temperature change resistor Re Will be changed 6.6 ohms to effect a change of control temperature of 1 F. If resistor Ra is wound with iron wire having a temperature coefiicient of about .0022 it will be desirable to cause it to change 3.3 ohms per degree Fahrenheit change in ambient temperature, thus the value of resistor Ra should be about 1500 ohms.

In the event the heater element should fail or the heater circuit should be otherwise broken the great increase in the resistance of this circuit will at one cause the relay to open. However, in order to positively retain the relay in its open condition I prefer to connect a resistor Rs of very high value across the relay terminals. Should the heater circuit open, the very small amount of current flowing through this safety resistor Rs will be sufiicient to impress a high voltage on the cathode of tube VI, which will cause the condenser CI to remain charged and thus hold the relay open. The current in this safety resistor has no eifect when the heater circuit is intact, since it amounts to less than milliampere. The condenser C2 is connected across the relay circuit to prevent the relay from chattering.

While I have shown and described one embodiment of my invention, I wish it to be understood that I do not desire to be limited to the details thereof as various modifications may occur to a person skilled in the art.

Having now fully described my invention, what I claim as new and desire to secure by Letters Patent is:

1. An electric temperature control system comprising an electrical heater having a known temperature coefiicient of resistance, an adjustable resistor, a bridge circuit including said heater and said resistor, an amplifier connected to said bridge circuit and controlled by changes in resistance in said bridge circuit, a relay controlled by said amplifier and adapted to energize said bridge circuit by selectively connecting said bridge circuit to a source of electrical power, timing means connected to said source of power and operable in response to de-energization of said bridge circuit by said relay to cause said relay to energize said bridge circuit after the elapse of a predetermined interval.

2. An electric temperature control system comprising a resistance element having a predetermined coefficient of resistance change with temperature, an adjustable resistor, a bridge circuit including said element and said resistor, relay means for selectively energizing said bridge circuit by connecting said bridge circuit toa source of electric power, an amplifier adapted to control said relay means in accordance with the relative resistances of said element and said resistor, when said bridge circuit is energized, and timing means connected to said source of power and operable to energize said bridge circuit in response to the elapse of a predetermined interval after said bridge is de-energized in response to operation of said relay.

3. In a controlled electrical heater system, a resistance heating element having a known variation of resistance with temperature, a bridge circuit including said element, a condenser, amplifying means connected to said bridge circuit for charging said condenser, resistor means for discharging said condenser at a predetermined rate, relay means for energizing said bridge circuit by connecting said bridge circuit to a source of electric power, and vacuum tube means for energizing said relay means in accordance withthe charge across said condenser.

4. A controlled electric heater system comprising a resistance heater element having a known relation of resistance to temperature, an adjustable resistor, a second resistance element having a high resistance compared with said resistance heater element, a bridge circuit including said heater element, said resistor and said second element, an amplifier connected to said bridge circuit, relay means to energize said bridge circuit by connecting it to a source of electric power, said amplifier servin to control said relay in accordance with the resistance of said heater element, and timing means to control said relay in response to a predetermined change in the resistance of said bridge circuit, said timing means serving to energize said bridge circuit after a predetermined lapse of time following the deenergization of said circuit by said relay.

5. In a controlled electric heating system, a resistance heating element having a known coefiicient of resistance with temperature, an ad justable resistor, a bridge circuit including said. element and said resistor, a relay including a coil, said relay also including contacts which close when said coil is energized, said contacts being adapted to carry current from a source of electrical power to said bridge circuit, a vacuum tubeincluding an anode connected to said coil, saidi vacuum tube also including a control grid and a. cathode, said cathode and said coil being adapted: to be connected to a source of electric power, a. condenser and a second resistor connected between said grid and said cathode, a second vacuum tube including a second anode connected to said grid, said second vacuum tube also including a second grid and a second cathode, a third resistor connected from said second cathode to said first cathode, a fourth resistor connected from said second cathode to said coil, a fifth resistor connected from said second grid to said first cathode, a third vacuum tube including a third anode connected to said second grid, said third vacuum tube also including a third cathode and a third grid connected to said bridge circuit.

6. In a controlled electric heating system, a resistance heater element having a known temperature resistance coefiicient, a second resistance element connected in series with said first element, an adjustable resistor having a relatively high impedance compared with said first element and said second element, a third resistance element connected in series with said adjustable resistor and having an impedance in ratio to said adjustable resistor substantially equal to the impedance of said first element in ratio to said second element, said adjustable resistor, said first element, said second element and said third element comprising a bridge circuit in balance at a predetermined temperature of said first element, an amplifier having a pair of input terminals, one terminal of said pair being connected to the junction of said first element and said second element, the other terminal of said pair being,

connected to the junction of said adjustable rejsistor andsaid third element, 'a power circuit "having a second pair'of terminals one of said I, second pair being connected to 'the junction of said first element and said thirdelement, the

other of said second pair being connected to the junction'of said second element and said adjustable resistor, a relay having a' coil and a switch changes in temperature in the blanket, a second resistance located outside'of said blanket and having a known temperature coeflicient of resistance so that the resistance of said, second resistance varies in response to ambient tem- *peratures, relay operated switching means for open-circuitin'g the heater circuit, means responc'ontact, said contact being adapted to energize said power circuit from a source of electric power in response to energization of said coil, vacuum tube means to selectively energize said coil, said vacuum tube means being in part controlled by said amplifier in response to energization of said [power circuit, and timing meansin part control.-

"ling said vacuum tube means in response to deenergization of'said bridge circuit upon opening said switch contact.

'7. In an electric temperature control system, a circuit having terminals A and 13' whereby it may be connected with an alternating current power 7 "line, a resistance heater in said circuit, a relay including a switch arranged to control the flow ofjcurrent through said heater, and an amplifier in said circuit including a vacuum tube the plate f'circuit of which is connected with terminal B through the winding of said relay, a second vacuum tube the plate circuitof which is connected with terminal A and includes a resistor and a condenser in parallel with said resistor, the plate circuit of said second tube and said condenser being connected with the grid of the first mentionedtube, a voltage divider connected with both terminals A and B, and the cathode of said second tube being connected with said divider be tween the ends thereof, a third vacuum tube the plate circuit of which is connected with terminal A and includes a resistor, means connected with said terminal B to impress a negative voltage on the cathode of said third tube and a smaller negative voltage on the grid thereof when terminal .Bis negative, saidgrid voltage being responsive to variations in the resistance of said heater, the grid of said second tube being'connected with the plate circuit of said third tube in advance of the resistor in said plate circuit, and means including a high resistance in parallel with said [switch'ior impressing a high positive voltage on the cathode of said third vacuum tube so as to cause'said condenser to remain charged and hold the relay open at all times in the event the resistance heater breaks.

8. An electric temperature control system comprising an electrical heater having a known temperature coefficient of resistance, an adjustable "resistor, a bridge circuit including said heater and said resistor, an amplif er connected to said bridge circuit and controlled'by changes in resistance in said bridge circuit, a relay controlled by said amplifier and adapted to energize said bridge circuit by selectively connecting said bridge circuit to a source of electrical power, timing means connected to said source of power and operable in response to de-energization of said bridge circuit for a predetermined period of time for causing said relay to energize said bridge circuit, and means for rendering said timing means inoperative in response to a break in the electric heater circuit;

9. In a. temperature control system for use with an electrically heated blanket, the combination including an electrical heater disposed within said blanket, said heater having a known tem- "perature coefficient of resistance so that the resistance of the heater changes in response to sive'to the resultant of the voltage drop across the heater and the second resistance for opencircuiting the heater when the temperature within the blanket reaches a predetermined value,

which predetermined value is changes inthe' ambient temperature, and timing influenced by means operable after the elapse of a predetermined interval to close said switching means to energiz said heater. 10. In a temperature control system for use with an electrically heated blanket, the combination including an electrical'heater disposed within said blanket, said heater having a known temperature coefiicient of resistance so that the resistance of the heater changes in response to changes in temperature in the blanket, a second resistarice located outside of said blanket and having a known temperature coefficient of resistance so that the resistance of said second resistance varies in response to ambient temperatures, relay operated switching means for opencircuiting the heater circuit, means responsive to the resultant of the voltage drop across the heater and the second resistance for open-circuiting the heater when the temperature within the blanket reaches a predetermined value, which predetermined value is influenced by changes in the ambient temperature, timing means operable after the elapse of a predetermined interval to 'close said'switching means to energize said heater, and means responsive to abnormal circuit conditions for making said timing means inoperative to prevent accidents.

'11. A control for an electric heating device having a, heating winding adapted to change its resistance with change in its temperature, relay means having contacts in circuit with said heating winding, means for intermittently momensaid winding, means for comparing said potential with a reference potential and thereby'deriving a control potential, an electron tube circuit, means applying said control potential to the in put of said circuit and thereby producin a change in the output current thereof, relay means having contacts in circuit with said heating winding and adapted to be operated by said output current to disconnect said heating winding from a supply source and means operable at the beginning of each heating interval to close said contacts.

'13. A control for an electric blanket having a heating winding with a high temperature coefficient of resistance comprising, in combination therewith, a resistor in series with said winding wherebythe potential drop across said resistor varies'in accordance with current flow through said winding, means comparing said potential drop with a reference potential and thereby deriving a control potential, an electron tube circuit, means applying said control potential to the input of said circuit and thereby producing a change in the output current thereof, relay means having contacts in circuit with said heating winding and adapted to be operated by said output current to disconnect said heating winding from a supply source and means operable at the beginning of each heating interval to close said contacts.

14. A control for an electric blanket having a heating winding with a high temperature coefficient of resistance comprising, in combination therewith, a resistor in series with said winding whereby the potential drop across said resistor varies in accordance with current flow through said winding, a resistance bridge network having said winding in one leg and said series resistor in an adjacent leg, means applying a potential to said network, an electron tube circuit, means applying the unbalance bridge potential to the input of said circuit and thereby producing a change in the output current thereof, relay means having contacts in circuit with said heating winding and adapted to be operated by said output current to disconnect said heating winding from a supply source and means operable at the beginning of each heating interval to close said contacts.

15. A control for an electric blanket having a heating winding with a high temperature coefiicient of resistance comprising, in combination therewith, a resistor in series with said winding whereby the potential drop across said resistor varies in accordance with current flow through said winding, a resistance bridge network having said Winding in one leg and said series resistor in an adjacent leg, means applying a potential to said network, an electron tube circuit, having an output tube normally biased to produce an output current, relay means having contacts in circuit with said heating windin and an operating coil connected to said output tube and adapted to be operated by said output current to close said contacts thereby connecting said heating winding to a supply source, means applying the unbalance bridge potential to the input of said electron tube circuit, said circuit being adapted to out off said normal output current in said output tube and thereby open said relay contacts in response to a predetermined increase in said heating winding resistance and means operable at the beginning of each heating interval to close said contacts.

FREDERICK J. HOOVEN.

REFERENCES CITED The following references are of record in tho file of this patent:

UNITED STATES PATENTS Number Name Date 1,183,814 Haagn May 16, 1916 1,694,264 Hull Dec. 4, 1928 1,776,901 Essex et al Sept. 30, 1930 2,084,186 Braden June 15, 1937 2,246,575 Coleman June 24, 1941 2,290,091 Brown et a1 July 14, 1942 2,455,379 McLennan Dec. 7, 1948 FOREIGN PATENTS Number Country Date 144,669 Great Britain Sept. 9, 1921 338,880 Great Britain Nov. 18, 1930 308,321 Germany Oct. 10, 1918 623,503 Germany Dec. 24, 1935 634,161 Germany Aug. 19, 1936 

